This invention relates generally to the basic oxygen process of melting and refining steel, and more specifically to a new mixed gas blowing technique in which iron oxide containing material is added to the melt during the blow.
Mixed gas blowing (oxygen and an inert gas) has been used in the BOF process for various reasons. U.S. Pat. No. 4,210,442, the disclosure of which is incorporated by reference, discloses a basic oxygen refining process in which argon is blown into the melt with the oxygen in order to prevent slopping caused by the evolution of carbon monoxide. As discussed in this patent, silicon, manganese, etc. are preferentially oxidized in the initial stage of the blow. After the metallic impurities are oxidized, more oxygen is available for reaction with the carbon in the melt and this results in increased evolution of carbon monoxide. It is at this stage of greater carbon monoxide evolution that slopping is likely to occur. According to U.S. Pat. No. 4,210,442, argon is then blown into the melt with the oxygen until slopping has stopped. In the illustrative examples of the process disclosed in the patent, the initial oxygen flow rate is maintained when injecting the argon.
U.S. Pat. No. 4,514,220 discloses a mixed gas blowing process used to make stainless steel in a top-blown vessel having a hot metal charge that forms the bath. The process involves introducing an inert gas beneath the surface of the charge throughout the blow. After the top blowing has commenced, an inert gas is injected with the oxygen while decreasing its flow rate. The bottom inert gas flow is used to produce stirring of the bath, while the top blown inert gas is used to dilute carbon monoxide formed during decarburization of the melt.
Mixed gas blowing also has been used to minimize oxidation of chromium in the production of stainless steel, to produce low nitrogen steels, and for degassing. In still another process disclosed in U.S. Pat. No. 5,374,297, a stream of inert gas is used when a carbonaceous fuel is introduced into the furnace with the oxygen. An inert gas is injected between the fuel and the oxygen stream to prevent premature combustion.
There has existed a need for a process that would facilitate the introduction of iron oxide containing material during the blow while maintaining optimum furnace performance. When iron oxide is added to the melt, the evolution of carbon monoxide is increased as the iron oxide is reduced. A conventional practice has been to reduce the oxygen flow when adding iron ore pellets to the melt. This reduction in the flow of oxygen during blowing has several disadvantages. The reduced jet momentum and penetration into the bath generates more iron oxide in the slag in addition to the iron oxides already introduced by the feed material. Also, the reduction of oxygen flow reduces the turbulence of the bath which in turn reduces the rate of melting and reaction of the iron ore or oxides, thereby adversely affecting performance of the BOF.